The present invention relates to a video printer for printing a video information onto paper, and more particularly to a preheat-type video printer which improves picture quality by preheating a thermal print head when storing data to a line memory.
A video printer prints a picture recorded by the instantaneous capture of a video signal, or an image which is reproduced on a monitor and was recorded by a still camera or the like. FIG. 1 is a block diagram of a video printer.
Referring to FIG. 1, a decoder 10 separates the R(red), G(green), and B(blue) analog signals from a video signal input from a first input port 5, and supplies them to an analog-to-digital (A/D) converter 20. The decoder 10 also separates the horizontal and vertical synchronous H sync and V sync signals from the video signal received via the first input port 5, and supplies them to a memory controller 70, which, upon receiving a memory instruction from a second input port 15, generates sampling pulses synchronized by the H and V sync signals from decoder 10 and supplies the same to A/D converter 20. The memory controller 70 also generates a write address and supplies it to a frame memory 30. The A/D converter 20 quantizes each of the R, G, and B analog signals from decoder 10 according to a sampling pulse train generated from memory controller 70, and encodes the quantized analog signals into digital signals. Since the video signal's transmission speed differs from its print speed, frame memory 30 stores R, G, and B digital signals from A/D converter 20 in its own storage locations corresponding to the write address from memory controller 70. When the video signal of a frame is fully stored in frame memory 30, memory controller 70 terminates the control for storing video signal.
Upon receiving a print instruction signal from a third input port 25, a print controller 80 simultaneously applies a read address to frame memory 30 and a write address to a line memory 31. At the same, print controller 80 also supplies a selection control signal to a memory selection switch 40 which sequentially selects one of the digital R, G and B signals from frame memory 30 in accordance with the selection control signal. This selected digital color signal output by memory selection switch 40 is supplied to a line memory 31 which stores a line of the digital color signal in its own storage locations corresponding to the write address output from print controller 80. Under control of print controller 80, each line of the digital color signal stored in line memory 31 is supplied to a controller 50.
The controller 50 sequentially converts the digital R, G, and B signals from line memory 31 to Y(yellow), M(magenta), and C(cyan) signals respectively and at the same time, performs resistance correction to reduce the resistive deviation of each thermal element of the print head and also performs color correction to compensate for the error of density conversion in accordance with the correlation between the properties of the particular paper in use and the amount of heat which the print head generates. Then, the controller 50 applies the converted and compensated Y, M, and C signals as thermal data to the print head of printer 60 to print one line at a time repeatedly, thereby completing the printing of one frame.
FIG. 2 is a timing diagram for carrying out video printing in a conventional video printer. The odd vertical scan period shown on the left in FIG. 2 corresponds to a line memory period during which line memory 31 stores one column data of a video signal to print onto paper. The even vertical scan period shown on the right in FIG. 2 is a printing period during which controller 50 processes the column data stored in line memory 31 to print the same onto paper by the print head of printer 60. During the line memory period, print controller 80 supplies a control signal to a power driving means, e.g., a brushless DC motor in the printer 60, which locks the motor during printing.
FIG. 3 is a block diagram of a conventional thermal printing apparatus for a video printer. Here, an input port 105 receives the digital video data from the frame memory 30 of FIG. 1 during the line memory period shown in FIG. 2. Referring to FIG. 3, line memory 31 stores and maintains one column data until one line has completed printing under the control of a controller 110. A command from controller 110 instructs the video data stored in line memory 31 to be supplied to comparator 120 which compares the video data with the count value from a gradation counter 100, and detects print samples by their gray levels. For the print samples, the video data is equal to the count value of gradation counter 100, establishing the output of comparator 120 as a logic HIGH. Comparator 120 supplies the print designation data for each detected sample to a shift register 191.
A temperature detector 140 detects the temperature of a thermal element, and supplies the detected temperature data to a gradation ROM 130. Gradation read only memory (ROM) 130 receives the temperature data from temperature detector 140 and the count value from the gradation counter 100 and reads out intensity/time data which is stored in locations corresponding to the count value from gradation counter 100 and temperature data from the temperature detector 140, and supplies it to a pulse-width modulated (PWM) signal generator 160. A reference pulse train of a constant frequency generated in a pulse generator 150 is supplied to the PWM signal generator 160 which uses the reference pulses to pulse-width modulate the intensity/time data from gradation ROM 130, and supplies the modulated PWM signal to a decoder 170 which in turn supplies a thermal element driving signal to a thermal element driver 193. During the pulse period of the PWM signal, the thermal element driving signal has constant voltage or current.
A shift register 191 serially receives the print designation data for each sample from comparator 120 according to the control clock from controller 110, and supplies each sample of the input print designation data in parallel to a latch 192. When a latch pulse is applied from a latch pulse generator 180 which is driven by controller 110, latch 192 transmits the print designation data for each sample in parallel from shift register 191 to a thermal driver 193. The thermal driver 193 applies thermal element driving signal from decoder 170 to thermal elements of the thermal element array 194 corresponding to the sample wherein the value of print designation data is "1" so that a thermal element array 194 performs the printing of the first gray level. When printing up to the 64th gray level according to the first gray level printing, the conventional thermal printing apparatus completes the printing of one column. A printing head 190 consists of the shift register 191, latch 192, thermal driver 193, and thermal element array 194.
Since the printing time of the conventional thermal printing apparatus for a video printer shown in FIG. 3 takes 1/60th of a second, the apparatus should increase the heat to the thermal elements in order to promote the intensity expression for 1/60th of a second. Thus, to increase the generated heat to the thermal elements, the instantaneous value of power according to the heat generating capacity must be increased, a large capacity semiconductor device must be employed as the semiconductor device of a thermal element driver, and the thermal elements must have high internal voltage.